1. Field of the Invention
This invention relates to a vibration wave motor, and more particularly to a structure for supporting a vibration member in a vibration wave motor.
2. Description of the Prior Art
A known vibration wave motor includes a vibration member and a movable member. The vibration member has an elastic metal member and electrostrictive elements fixed to the elastic member. When an AC voltage for driving is applied to the electrostrictive elements, elliptic vibrations are produced on a surface of the elastic metal member. The movable member is frictionally contacted to the vibration member so as to be rotatably driven by the elliptic vibrations. In fixing the vibration member to a motor case, i.e., a stator, it is necessary to reduce to a minimum the transmission of vibration from the vibration member to the stator.
For that purpose, various supporting structures have been proposed. One of the most commonly used supporting structures comprises a means for supporting a vibration member through an absorber made of rubber, etc., as described in U.S. Pat. No. 4,513,219. Further, as described in U.S. Pat. Nos. 4,634,915 and 4,649,311, it has been proposed to provide a vibration member with auxiliary vibration members to be used as supporting members, in which a standing wave is set up with one of its nodes fixed at a stator or a motor case.
However, the conventional supporting means as described in U.S. Pat. No. 4,513,219 has the following problems:
(1) Since rubber and the like are used for supporting, the position of the vibration member is unstable and cannot be fixed.
(2) In order to drive a vibration wave motor, the movable member must be press-contacted to the vibration member with a strong pressure. If the pressure is intensified, however, rubber and the like are subjected to elastic deformation and their vibration absorbency is reduced, so that the efficiency of the motor is lowered and noise is produced.
(3) Since rubber and the like are not subjected to a uniform elastic deformation, the distribution of the pressure between the vibration member and the movable member is not uniform, so that the motor rotates unevenly and noise is produced.
Further, the supporting means as described in U.S. Pat. Nos. 4,634,915 and 4,649,311 has the following drawbacks:
(1) Although the position of the vibration member can be fixed, a supporting portion must be relatively long in order to set up a standing wave in the supporting portion, so that a substantial space is necessary for the supporting portion. As described in U.S. Pat. Nos. 4,634,915 and 4,649,311, the relationship between the frequency and length of the auxiliary vibration member (supporting member) is represented by the following equation, assuming that the length of the auxiliary vibration member is sufficiently longer than its thickness and its sectional shape is uniform: ##EQU1## where
1 is the length of the auxiliary vibration member,
.beta. is a constant corresponding to the order of vibration,
E is the modulus of longitudinal elasticity of the auxiliary vibration member,
I is a sectional secondary moment of the auxiliary vibration member,
.rho. is the density of the auxiliary vibration member,
A is a sectional area of the auxiliary vibration member, and
.omega. is an angular frequency. Assume that the auxiliary vibration member is a plate having the thickness h and breadth b, the cross section of which is a rectangle. Since I=bh.sup.3 /2, the above equation can be transformed as follows: ##EQU2## where f is a vibration frequency of the auxiliary vibration member. If the auxiliary vibration member is a rectangular plate of stainless steel having the thickness of 1 mm, 1 is 0.54 mm in case of the fundamental vibration. In this case, the rigidity of the member is so high that bending vibration is hardly produced. In order that the above equation can be established, the length 1 must be sufficiently longer than the thickness of the plate. Therefore, it is necessary to reduce the thickness of the plate, lengthen the length 1 and produce vibration in a mode of higher order. This inevitably requires a large space for the auxiliary vibration members.
(2) In order to apply a stable pressure, an urging member such as a spring is necessary in addition to the supporting member.